160 DISEASES OF CROPS. 



microscope, and it is necessary to have good lenses to 

 do satisfactory work. The powers chiefly required for 

 the entomological portion of the subjects herein treated 

 are the following : — 



Zeiss's Objectives. English Objectives. 



a. or 3 inches focal distance, 



aa. ,, 1 ,, }t 



ii. ti i .. M 



C. ,, i I, II 



These lenses magnify from about 20 to 250 linear dia- 

 meters with the lowest eye-piece (A, or No. 1). For the 

 examination of fungi and diseased vegetable tissues (in 

 addition to the above objectives) Zeiss's E and H, or an 

 English i inch and -^ inch, are necessary. To make out 

 the nuclei and cilia of the smallest zoospores, etc., it is 

 essential to be provided with a still higher power, such 

 as Zeiss's -jV oil-immersion lens, which the author can 

 thoroughly recommend. It has a perfect definition, and 

 everything there is to be seen can be made out with this 

 lens. 



The Eye-piece. — Zeiss's Huyghenian eye-pieces Nos. 

 1, 3, and 5, or the A, 0, and D of English makers, are very 

 useful for the economic biologist. 



Illumination. — Daylight is the best light to use for 

 the study of parasitic fungi and other transparent objects. 

 But in the winter and for opaque objects (small beetles 

 and other insects) a paraffin lamp is essential. " The one 

 thing requisite in a lamp is that the flame is steady; this 

 depends on the wick fitting properly." For the examina- 

 tion of opaque objects it is necessary to be provided with 

 a stand condenser.' 



' A description of the stand and sub-stage condensers will be found 

 in manuals devoted to the microscope. 



